[Diagnostics] Remove the C++ formatter for the "Swift" diagnostic style

We'll be using the new swift-syntax diagnostic formatter in the near
future, as it is nearly available on all host platforms. So, remove
the C++ formatter that did source-line annotation, falling back to the
"LLVM" style when swift-syntax is not compiled in.

Author: DougGregor

include/swift/Basic/SourceManager.h

@@ -410,8 +410,6 @@ class SourceManager {
                                SourceLoc();
   }
 
-  std::string getLineString(unsigned BufferID, unsigned LineNumber);
-
   /// Retrieve the buffer ID for \p Path, loading if necessary.
   unsigned getExternalSourceBufferID(StringRef Path);
 

include/swift/Frontend/PrintingDiagnosticConsumer.h

@@ -27,7 +27,6 @@
 #include "llvm/Support/Process.h"
 
 namespace swift {
-class AnnotatedSourceSnippet;
 
 /// Diagnostic consumer that displays diagnostics to standard error.
 class PrintingDiagnosticConsumer : public DiagnosticConsumer {
@@ -38,10 +37,6 @@ class PrintingDiagnosticConsumer : public DiagnosticConsumer {
   bool DidErrorOccur = false;
   DiagnosticOptions::FormattingStyle FormattingStyle =
       DiagnosticOptions::FormattingStyle::LLVM;
-  // The current snippet used to display an error/warning/remark and the notes
-  // implicitly associated with it. Uses `std::unique_ptr` so that
-  // `AnnotatedSourceSnippet` can be forward declared.
-  std::unique_ptr<AnnotatedSourceSnippet> currentSnippet;
   // Educational notes which are buffered until the consumer is finished
   // constructing a snippet.
   SmallVector<std::string, 1> BufferedEducationalNotes;